Space Settlements

The topography of spacepp. 9–10

Each planet or moon sits at the bottom of a gravity well, and Earth's is 22 times deeper than the Moon's. The five Earth–Moon libration points are shallower wells. L1, L2 and L3 are saddle-shaped and unstable; L4 and L5 are bowl-shaped, and when the Sun is included, objects near them follow stable orbits with a period of about a month (pp. 9–10).

Distance and effort are not the same thing. Going from low Earth orbit to lunar orbit takes about 5 days but needs a velocity change of 4,100 m/s, only 300 m/s more than reaching geosynchronous orbit. Most of the effort near Earth goes into the first 100 km (p. 10).

From the report · Fig. 2-2, p. 10

Solar radiation: an abundant and essential source of energyp. 11

A square metre facing the Sun receives 1,390 W, nearly twice the 747 W maximum at Earth's surface, and almost 7.5 times the average over the whole Earth once night and slant are counted. Converted at 10 percent and sold at $0.012/kWh, one square kilometre of sunlight would earn more than $14 million a year (p. 11).

Matter in space: a major resourcepp. 11–18

The Moon is the chosen source: close by, in a shallow well, and rich in aluminium, titanium, iron, oxygen and silicon, though poor in hydrogen, carbon and nitrogen, which must come from Earth. Asteroids, some possibly carrying water and carbon, are flagged as a likely later resource (pp. 11–12).

Meteoroids are judged a small hazard. A square kilometre is crossed by a 1-g meteoroid about once every 10 years and by a 100-g one about once in 5,000 years. A 1-m hole, expected once in 10 million years, would take at least a day to let out 60 percent of the air (pp. 12–13, Appendix A, Table 2-1).

Ionizing radiation is the major threat. Unshielded, galactic cosmic rays deliver about 10 rem a year, and heavy nuclei such as iron destroy columns of cells along their tracks. Solar flares of the 1956 class could give many times a fatal dose to an unprotected person. A thin shield of a few tonnes per square metre can increase the dose, to as much as 20 rem a year, because of secondary particles. For reference the report cites 5 rem/yr for radiation workers, under 0.5 rem/yr for the general population, and about 0.1 rem/yr as a typical natural background (pp. 13–15, Appendix B).

Weightlessness: pseudogravity is neededpp. 21–22

Spaceflight so far had shown bone decalcification of 1–2 percent a month with no sign of levelling off, along with hormone and fluid changes and a difficult readaptation to 1 g. With no data between 0 and 1 g, the study set its criterion at 0.9 to 1 g (pp. 21–22).

Rotation brings Coriolis forces. Someone jumping 0.55 m off the mid-deck of the torus lands about 5.3 cm to one side, and head movements out of the plane of rotation can cause motion sickness. People can adapt to about 3 rpm, but adaptation may suffer from frequent changes of rate, for example by commuters to zero-g workplaces. The study therefore set 1 rpm as the upper limit for living quarters (p. 22).

From the report · Fig. 3-1, p. 22

Atmosphere: less is enough; food and waterp. 23

Breathing needs oxygen at a partial pressure near sea level's 22.7 kPa, within about ±9 kPa. Nitrogen is added as an inert buffer, perhaps at 26.7 kPa; carbon dioxide stays below 0.4 kPa; temperature near 22 °C at about 40 percent humidity. The result is that people can live comfortably at a total pressure well below one atmosphere. A diet for heavy work is about 3,000 Cal a day, including 2 kg of water (p. 23).

Environmental design, space needs, and small size and isolationpp. 23–28

The habitat should avoid motel banality: long lines of sight, parts hidden from each other, living things, contact with the outside, and an interior that the colonists finish themselves from modular parts. The literature suggests at least 40 m² of projected area per person, where projected area is floor area projected onto a plane perpendicular to the pseudogravity (p. 24). A land-use inventory (Table 3-2) gives 47 m² and 823 m³ per person for the community and another 20 m² and 915 m³ for agriculture (p. 25).

A town of 10,000 cannot be self-sufficient: the report cites estimates that a full range of commercial services needs 100,000 to 200,000 people. Trade with Earth, electronic communication, a route home for those who want it and a governance able to protect a fragile habitat are all treated as design requirements (pp. 25–28).

Summary of design criteriap. 28

The shape of the habitatpp. 39–43

The habitat must hold about 670,000 m² of projected area, which is 67 m² for each of 10,000 people (p. 40, Appendix B). To hold air economically the shell must be a membrane in pure tension, so it must be a closed, doubly symmetric shell of revolution. That leaves four basic shapes and their composites: sphere, cylinder, torus and dumbbell (pp. 40–41). Aluminium is assumed throughout.

From the report · Fig. 4-2, p. 40

Pseudogravity at radius $R$ for a spin rate $\omega$ is $g={\omega }^{2}R$. At 1 rpm ($\omega =0.105$ rad/s), 1 g requires a radius of 895 m, so only systems with radii of rotation greater than 895 m can give 1 g below 1 rpm (p. 41):

From the report · Fig. 4-4, p. 42

Mass decides between shapes. A cylinder like O'Neill's C-3 would weigh about 42,300 kt and hold 800,000 people; a 895-m sphere about 3,545 kt for 75,000. A torus can choose its tube radius independently of its spin radius, so a torus of 830 m and 65 m radii needs only about 150 kt. For a given radius of rotation, cylinders and spheres need about four times as much mass per unit of projected area as a slim torus (pp. 41–42, Appendix C).

Composite structures are lightest of all, but were set aside largely on architectural criteria of environmental perception: short sight lines and dense internal cables would feel oppressive (p. 42). The single torus is adopted for its balance of low mass, large volume, possible incremental construction, a clear circulation pattern, zero-g access at the hub and a clear visual horizon (p. 43).

Shieldingpp. 43–46

Active shields. A magnetic field can turn away particles below a cutoff energy. A cutoff of 0.5 GeV per nucleon, with about 10 kt of structure, would actually raise the dose to about 20 rem/yr through secondaries made in the structure; a 10–15 GeV cutoff would work but needs too much structure to hold the superconducting coils apart. Electrostatic shielding would need about 10 billion volts; a plasma shield is promising but needs research (pp. 43–44, Appendix D).

Passive shield. Earth's atmosphere provides about 10 t/m². Half of that would bring cosmic rays down to 0.5 rem/yr, and because many particles cross the shield at a slant, 4.5 t/m² suffices around the torus: 9.9 million tonnes in all (pp. 44, 46). Appendix E derives the thickness from a conservative formula for a thick shield of lunar soil:

So much mass cannot spin at 1 rpm without overstressing it, so the shield is a separate shell that is stationary or turns very slowly. The tube therefore moves past the shield's inner surface at 87 m/s, a metre or two away. The study group's consensus was that keeping them aligned would not be difficult in principle, though it was not studied in detail (p. 46).

What if the criteria change?pp. 46–48

The chapter then relaxes the criteria to show how sensitive the shape is to them. Moving the farms outside the shield and cutting the projected area from 47 to 35 m² per person shrinks every option (Table 4-2). Allowing 0.85 ± 0.15 g at 1.9 rpm instead of 0.95 ± 0.05 g at 1 rpm reduces the radius of rotation to 236 m (p. 47).

With the smaller radius a single torus is too small and a cylinder far too large, but a sphere holds exactly 10,000. The resulting Bernal sphere, with farms in thin rings outside the shield, needs 300,000 t less shielding than its nearest rival and millions of tonnes less than the rest, but 3–4 times the atmosphere and 2–4 times the structure (p. 48). Which shape wins depends on the relative cost of shielding, structure and atmosphere (p. 48).

From the report · Fig. 4-7, p. 48

Fabrication techniques; the people in the colonypp. 48–52

The shell is built outward from the hub, through the spokes and around the torus, by space tunneling: movable end caps advance along the tube so that workers assemble prefabricated parts in shirt-sleeve conditions. Metal-vapour deposition onto a rotating form is suggested for study. Inside, a light tubular frame with non-load-bearing walls and prepackaged bathrooms is preferred (pp. 48–49).

The population is designed too. It grows from about 2,000 construction workers who bring one to three family members each, and is weighted toward ages 21–44 and toward workers, like a frontier population. About 61 percent of the workforce, or nearly 44 percent of all colonists, produce for export (pp. 51–52). Governance is expected to start as familiar corporate or governmental management and evolve as a community identity forms (p. 52).

Life supportpp. 52–54

Food from Earth at 1.67 t per person per year would cost about $7 billion a year to ship, so food is grown in space, using familiar crops and animals rather than algae. All wastes are recycled by wet oxidation at 1,500 lb/in² and 260 °C, which in about 1½ hours yields sterile water, carbon dioxide and ash. The atmosphere is set at the minimum pressure that meets the criteria (pp. 52–53):

Satellite solar power stations; where the colony should be locatedpp. 54–55

The study found no alternative to building satellite solar power stations as the colony's main business, and the authors note that the design depends entirely on this one enterprise, with smaller manufactures and new colonies as additions. Both Woodcock's turbogenerator design and Glaser's photovoltaic design were considered; the former was favoured as current technology (p. 54).

The habitat goes near the Moon but not on it: the lunar surface gets full sun only part of the time, offers no choice of gravity, and is a poor place to ship power stations from. Orbits close to Earth or Moon suffer eclipses; L1–L3 need station-keeping; L5 was chosen over L4 somewhat arbitrarily (pp. 54–55).

Mining, transport, and processing in space; the transport systempp. 55–62

About 10 million tonnes are needed, nearly all from the Moon. Processing happens at L5 rather than on the Moon, because the shield needs more mass than the plant's slag anyway, and because at L5 there is constant sunlight, efficient radiators and no weight (p. 56). Aluminium is extracted from anorthosite by melt-quench-leach and electrolysis of aluminium chloride; titanium from ilmenite is recommended for further study; glass is made from lunar silica (pp. 56–59).

Launch is by electromagnetic mass driver: superconducting buckets carry tens of kilograms of compacted soil, accelerated at about 30 g, with release velocity controlled to better than $10^{-3}$ m/s at one to five buckets a second. A hydrogen gas gun was considered and kept for further study (p. 59). The stream goes to an active catcher at L2 rather than L5, to keep it away from the colony, to shorten the shot, and to keep the launcher on the near side (p. 59).

From Earth, about a million tonnes must be launched over 6–10 years. A shuttle-derived heavy-lift vehicle with four solid boosters carries 150 t for under $20 million per launch; beyond orbit, tugs use the space shuttle main engine, with oxygen from lunar ore at L5 (pp. 60–62, Figs. 4-14 and 4-15).

The habitat at L5pp. 87–103

From the report · Fig. 5-1, p. 88

The tube is 130 m in diameter, the wheel about 1,800 m. Glass on aluminium ribs covers a third of the tube's surface and admits light downward onto farms and homes; the rest is aluminium plate. The separate shield is 1.7 m of fused lunar-soil bricks with a gap of about 1.5 m. Over the windows it becomes mirrored chevrons that pass light by successive reflections but stop cosmic rays (p. 89, Appendix K).

From the report · Fig. 5-2, p. 88

Sunlight in space is 1,400 W/m², but homes need about 200 W/m² and fields up to 1,000. Individually steered secondary mirror segments provide a day–night cycle for homes and continuous light for some crops (p. 89). If the shield served as the reaction mass for spin-up, it would counter-rotate at about 0.07 rpm (p. 89).

From the report · Frontispiece, ch. 5

Housing for 10,000 occupies 43 ha by terracing up the curved walls and putting shops, light industry and machinery on levels below the central plain. The torus offers 678,000 m² of projected area and encloses 69 million m³; the 18 million m³ actually needed is only 26 percent of it, and the rest makes the colony feel less crowded (p. 91).

The shell. The aluminium shell must hold 50 kPa of air, its own spin load and 530,000 t of soil, water, buildings and people. Appendix A sizes the skin of a stressed-skin torus in the meridional and hoop directions:

Because a third of the surface is ribbed window, the report takes the shell mass as two-thirds stressed skin plus one-third fully ribbed (229,000 t), or 156,000 t. The windows, 2.8 cm thick across 0.5 m between ribs, add 48,000 t of glass (pp. 94, 112).

Agriculture. Three farm zones alternate with three residential areas, each zone sealed so that it can run hot, humid and CO₂-rich. Tiers of fish ponds, grain fields and livestock triple the cropland. Lunar soil foamed into a light growth matrix 0.3 m deep, year-round sun, extra CO₂ and control of pests and weather are expected to give about 10 times the yield of a typical American farm. The colony feeds 10,000 people from 61 ha (pp. 95–98).

Water, waste and heat. Drinking water is condensed from the humid air, 95 percent of it transpired by plants. The water system supplies 25 times the metabolic need, recycles 250 kg per person per day for waste transport, and still uses only 75 percent of U.S. domestic water use (p. 101). Electricity is set at 3 kW per person, twice the U.S. figure, to cover recycling. The radiator sheds 131 MW: at 280 K a black body radiates 348 W/m², so a 60-percent-effective radiator needs 6.3 × 10⁵ m², and 50 percent more for peak loads gives 9.4 × 10⁵ m² and 2,400 t (pp. 102–103).

Production at L5pp. 103–106

Heavy processing sits in an extraction plant 10 km south of the hub with its own solar furnaces and a 200-MW solar power station. Its aluminium line melts lunar soil at 2,000 K, quenches it to glass, leaches it with sulfuric acid and ends with electrolysis of aluminium chloride; it produces about 150 t of aluminium a day, or 54 kt a year, with a plant of 7,620 t (pp. 103–105, Tables 5-9 to 5-12).

Building a 10-GW thermal power station is estimated at 2,950 man-years of off-Earth labour (Table 5-7, p. 104). Satellite repair, metal foams and single crystals are noted as smaller lines of business (p. 104).

The lunar basepp. 106–108

The mine is at the Apollo 16 site in the Cayley area, where Apollo samples had the most aluminium. Supplying a million tonnes a year means scooping an area about the size of eight football fields each year at about 4 t a minute. 150 people live there on two-year tours, under 5 m of lunar soil (pp. 106–107).

The mass launcher fires five 10-kg payloads a second at lunar escape velocity, 2,400 m/s, with the launch velocity held to within $10^{-4}$ m/s along track and $10^{-3}$ m/s across. Buckets accelerate at 288 m/s² over 10 km, release their loads, decelerate and return. At a 70 percent duty cycle it launches 1.1 million tonnes a year; two launchers and 220 MW of nuclear power keep it running through the lunar night (pp. 107–108, Appendix F).

From the report · Fig. 5-19, p. 108

The mass catcher at L2; home to Earthpp. 108–110

Payloads arrive at L2 as sintered blocks 0.20 m across, each 10 kg, at 200 m/s. A light net steered by three cables can be placed anywhere in a triangle 1 km on a side; the catcher weighs 220 t. A rotary pellet launcher, a fast-spinning 30-m tube that flings rock pellets at up to 4,000 m/s, keeps it pointed and absorbs the stream's push of about 2,000 N, using 5 percent of the received mass. Ore carriers take about two months from L2 to L5 (pp. 108–110).

The visitor leaves after two months and 750,000 km, wondering whether the colony will flourish: only time will tell (p. 110).

Key assumptions

• Physiological criteria set conservatively in the absence of data: 0.9–1 g, at most 1 rpm and at most 0.5 rem/yr for everyone, including children (pp. 22, 28, 179).
• Near-term technology only, with aluminium as the structural material: a working stress of 200 MPa, two-thirds of yield, as for large civil structures (pp. 4, 63).
• A passive shield of 4.5 t/m² that is separate from the rotating torus and stationary or slowly turning, sized by the conservative Appendix E dose formula with a factor-of-2 margin (pp. 44, 46, 125).
• Agricultural yields about 10 times those of a typical American farm, from continuous light, extra CO₂ and controlled conditions (p. 98).
• A population of 10,000 weighted toward working ages, drawn from Western industrialized nations, with 44 percent producing for export (pp. 51–52).
• Labour productivity taken from 1975 terrestrial industry factors, adjusted for space; plants producing 8.3 times their own mass per year; no bootstrapping beyond pilot plants (pp. 103, 143–144).
• Costs at $5,000 per kg for development and $500 per kg for purchase, a heavy-lift launch of 150 t for under $20 million, and satellite solar power as the economic product, with electricity benefits valued against 14.1-mil nuclear power (pp. 60, 145, 155).

Where further work would help

• Gravity and spin limits. The torus's 895-m radius follows directly from 1 g at 1 rpm. Table 4-2 shows that relaxing these to 0.7–1.0 g at 1.9 rpm cuts the radius to 236 m and favours a sphere, so long-term data on partial gravity and on adaptation to faster rotation would show how firmly the chosen shape is anchored (pp. 47–48, 179).
• Radiation limits and shielding. The 9.9-million-tonne shield is most of the colony's mass. Its size depends on the dose limit, on the exponential approximation of Appendix E (stated to be good to perhaps a factor of 2), and on slant-path geometry; a fuller transport calculation and biological data on chronic dose would show how much of that mass is required (pp. 46, 125, 179).
• Relative costs of shielding, structure and atmosphere. The authors note that the choice among torus, sphere and composite shapes turns on these relative costs, which were not determined in the study (p. 48).
• Closed ecology and farm yields. The 61–63 ha of farmland rests on yields that the report notes had not yet been demonstrated in a closed loop at scale; small and then large closed-system demonstrations were recommended before colonization (pp. 98, 179).
• Lunar materials chain. Lunar glass of window quality, the aluminium and titanium extraction routes, the mass launcher's precision and the L2 catcher were all at the concept stage; the report recommends laboratory and pilot-plant work, a scaled launcher prototype, and much greater study and eventual testing in space of the catcher (pp. 58, 179–180).
• Shield–torus clearance. Keeping a 1.5 m gap between a tube moving at 87 m/s and a stationary 10-million-tonne shell was judged feasible in principle but not studied in detail (pp. 46, 181).
• Economic sensitivity. The benefit-to-cost ratio moves from 1.02 to 1.5 between 10 and 8 percent discount rates, and the first colony's cost from $196.9 billion to $112.7 billion with an earlier second-generation shuttle; productivity in space, launch costs and the electricity market would each need wider ranges explored (pp. 155, 180).